This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. The subject relates to fuel preparation for molten salt breeder reactors, and more particularly to the reconstitution of spent molten fuel salt after fission product removal.
In a Molten salt Breeder Reactor, fissionable material is present in the form of a molten fuel salt which continuously flows in a primary circuit through the reactor core to a heat exchanger and back to the core. The fuel salt is a composition of mutually soluble metal fluorides selected for their nuclear and physical properties. Of particular interest is a fuel salt consisting of a carrier salt of LiF--BeF.sub.2 --ThF.sub.4 (72--16--12 mole %) and UF.sub.4 present in amounts of 1--4% by weight.
During the course of reactor operation, fission products including rare earths and bred-in protactinium build up in the fuel salt and adversely affect the nuclear properties of the fuel. In order to more efficiently operate the reactor, the level of neutron poison fission products must be kept at a minimum. This is accomplished by continuously removing spent fuel from the primary circuit, processing it to remove fission products, and returning the reprocessed molten salt to the primary circuit. It is desirable for safety and economy that the fuel processing plant be a component of the reactor itself and that the salt be kept in the molten state throughout the processing system.
While fused molten salts are often spoken of as mixtures of several metal salts, they are actually a special class of liquid composed entirely of positively and negatively charged ions. Some molten fluorides are known to present material compatability problems. UF.sub.5 is so corrosive that fluoride fuel salts containing the U(V) ion must be handled with gold equipment.
Pa and rare earth fission products can be readily removed from the molten salt stream by contacting the spent fuel salt with liquid bismuth containing a reductant (Li or Th). Since uranium is also taken up from the salt in this manner, it is more economical that the uranium component be removed by fluorination prior to rare earth removal and returned to the fuel salt. The fluorination step is accomplished by passing F.sub.2 gas countercurrently through the molten salt in a fluorination zone to oxidize dissolved UF.sub.4 in the fuel salt to gaseous UF.sub.6, which exits the fluorination zone with unreacted F.sub.2.
The most efficient way to reconstitute the fuel salt is to return the uranium to the fuel salt stream by passing the UF.sub.6 -F.sub.2 gas stream directly from the fluorination zone through fuel salt containing dissolved UF.sub.4. The UF.sub.6 --F.sub.2 stream reacts with the UF.sub.4 by the following reactions: EQU UF.sub.4 (d) + UF.sub.6 (g) .THETA. 2 UF.sub.5 (d) 1 EQU UF.sub.4 (d) + 1/2 F.sub.2 (g) .THETA. UF.sub.5 (d) 2
The salt which now contains UF.sub.5 is then treated with hydrogen to reduce the UF.sub.5 to UF.sub.4 by the following reaction: EQU UF.sub.5 (d) + 1/2 H.sub.2 (g) .THETA. UF.sub.4 (d) + HF(g) 3
With respect to the reactions herein described, (d) refers to the dissolved phase and (g) refers to the gaseous phase.